what will be source and destination MAC address

If router A want to send data to router B, what will be the source and destination MAC address.

Because the example was shown with Serial interfaces between the routers there would not be a an Ethernet Frame with a MAC address; however, if the routers were connected via Cat5 (or other) cable, it would use Ethernet and a MAC address, correct?

Ethernet uses MAC address at the DataLink Layer. Serial Links will use differnet encapsulation which will depend on which serial protocol being used.

PPP for example would use LCP (Link Control Protocol). Also (Network Control Protocol) NCP's which vary depending on which transport is being used (i.e. Ethernet/AppleTalk/etc..)

Cheers

Thanks Smurf.

Not a problem